1. Field of Invention
The present invention relates to a semiconductor device, and more generally to a lateral avalanche photodiode (APD) structure.
2. Description of Related Art
An avalanche photodiode is widely applied in optical communication due to the high speed and internal gain thereof. A conventional avalanche photodiode has a vertical structure, and the dopant concentration of each layer can be directly designed to obtain the optimal avalanche result. However, the method of forming such vertical avalanche photodiode cannot be integrated with the current commercial process (e.g. CMOS process). Therefore, the applicability is significantly limited.
Nowadays, the avalanche photodiode fabricated by a CMOS process becomes popular in the application of optical communication at near-infrared wavelengths (e.g. 850 nm) due to its low cost and possible integration with receivers. However, the light penetration depth of silicon material at 850 nm is greater than 10 μm, resulting in low responsivity of the avalanche photodiode. Moreover, carriers generated from the silicon substrate slowly diffuse to be collected and greatly affect the response performance.
FIG. 1 schematically illustrates a partial perspective view of a conventional avalanche photodiode structure fabricated by a CMOS process. A conventional avalanche photodiode structure 10 includes a P-type substrate 12, a plurality of P-type well regions 14, a plurality of N-type well regions 16, a plurality of S/D doped regions 18, a plurality of shallow trench isolation (STI) structures 20 and an electrode layer 22. The P-type well regions 14 and the N-type well regions 16 are arranged alternately and separated from each other by the STI structures 20. The S/D doped regions 18 are disposed in the P-type well regions 14 and the N-type well regions 16. The electrode layer 22 is disposed on the P-type substrate 12 and electronically connected to the P-type well regions 14 and the N-type well regions 16. However, in the conventional avalanche photodiode structure 10, a reverse bias is applied to PN diodes, and the PN diodes are operated at the breakdown regions. In other words, the conventional avalanche photodiode structure 10 is not like a typical avalanche photodiode including an avalanche area and a light receiving area, so that the response speed and bandwidth thereof cannot meet the customer's requirements.